The present invention relates to a ball screw.
A ball screw is a linear guide apparatus which is composed of a screw shaft, a nut and a plurality of balls. In the outer peripheral surface of the screw shaft and in the inner peripheral surface of the nut, there are formed spiral-shaped ball rolling grooves. These two spiral-shaped ball rolling grooves cooperate together and define a raceway along which the balls are allowed to roll. In case that the balls roll on the raceway while being loaded, the nut moves linearly with respect to the axis of the screw shaft. That is, the spiral-shaped ball rolling grooves of the screw shaft and the nut provide a ball rolling raceway. Also, in the case of a ball screw of a tube type using a tube-shaped ball circulating passage, there is disposed a tube which allows the balls to circulate from one end side of the raceway to the other end side thereof.
FIG. 3 is a partially sectional view of a conventional ball screw of the tube type. In FIG. 3, a one-dot chained line A is a straight line which connects a central point O3 of a ball 3a with an axial core center O of the ball screw. The central point O3 of the ball 3a is presented at a ball scooping position where a ball 3 shifts from a loaded rolling portion formed within a raceway 6. Here, the loaded rolling portion means a portion where the balls rolled within the raceway in the loaded manner, are released from the located manner. Also, another one-dot chained line B is a straight line (a reference line) which is perpendicular to the rising direction of a tube 7 from the raceway 6 and passes through the axial core center O.
As shown in FIG. 3, in the conventional ball screw of the tube type, the ball scooping position is setted upwardly of the reference line B (that is, on the rising side of the tube 7). An angle xcex8 of the ball scooping position with respect to the reference line B is setted at an angle in the range from 15xc2x0 to 36xc2x0. Also, the tube is disposed in such a manner that an axis 71a of a rising portion 71 of the tube 7 coincides with a line 60a corresponding to a ball center locus 60 of the raceway 6.
On a screw shaft 1 side of the rising portion 71, there is disposed a tongue 72 which can be inserted into a ball rolling groove 4 of the screw shaft 1. Normally, the tongue 72 is projected downwardly of the end face of the tube 7 and thus the tongue 72 exists in the advancing direction of the ball 3. With this structure, the ball 3, which has reached at the ball scooping position, can be contacted with and picked up by the tongue 72. Then, the ball 3 is allowed to move through a space defined between the tongue 72 and a hole 21 formed in a nut 2, and can be then guided into the tube 7. The hole 21 is formed such that it continues with the inner surface of the rising portion 71 of the tube 7.
However, in the above-mentioned conventional ball screw of the tube type, there are following problems. That is, the advancing direction of the ball 3 is changed at a boundary point D between the ball rolling groove 5 of the nut 2 and hole 21 as well as at a leading end point E of the tongue 72. In this bout, noises, which are generated when the balls 3 pass through these points, become loud. Also, not only noises which are generated when the balls 3 moving toward the tube 7 are butted against the tongue 72, but also noises which are generated when the balls 3 are butted against the screw shaft 1 after the balls 3 have moved from the tube 7 to the raceway 6, are loud. Further, the collision of the balls 3 with the tongue 72 applies large stresses to the tongue 72 repeatedly and thus, especially when the ball screw is rotated at a high speed, there is a fear that the tongue 72 can be damaged or broken.
Therefore, in the conventional ball screw of the tube type, there is a need for reduction in the vibrations and noises thereof. As measures to deal with the large vibrations and noises, conventionally, there are employed a method of increasing the thickness of the tube and a method of increasing the strength of the tongue through a heat treatment. However, with use of these methods, the manufacturing cost of the ball screw becomes high.
The present invention aims at eliminating the drawbacks found in the above-mentioned conventional ball screw of the tube type. Accordingly, it is an object of the present invention to provide a ball screw, which reduces the vibrations and noises effectively and also is manufactured at a low cost over the conventional ball screw.
In attaining the above object, according to the present invention, there is provided a ball screw, comprising: a screw shaft including a spiral-shaped ball rolling groove formed on an outer peripheral surface thereof; a nut including a spiral-shaped ball rolling groove which is corresponded to the spiral-shaped ball rolling groove formed on the screw shaft so as to form a raceway therebetween and is formed in an inner peripheral surface thereof; a plurality of balls respectively disposed within the raceway defined by the two spiral-shaped ball rolling grooves; and a tube for circulating the balls from one end side of the raceway to the other end side thereof and including rising portions which are disposed in a rising direction from the raceway, the tube being connected to the one end side of the raceway and the other end side thereof via the rising portions respectively; wherein an architectonic ball scooping position, where the balls are released from a loaded rolling portion formed in the raceway, is positioned on or in adjacent to a reference line, which is perpendicular to the rising direction of the rising portion and passing through an axial core point of the screw shaft, and wherein the tube is disposed in such a manner that the axis of the rising portion presents at a nut-side outside of a line corresponding to a ball center locus of the raceway in the diameter direction. in addition, the architectonic ball scooping position is also positioned downward of the reference line and positioned on the opposite side to the rising portion.
In the ball screw according to the present invention, preferably, the nut-side inner surface of the rising portion of the tube may be connected with the ball scooping position of the ball rolling groove of the nut by a surface existing outside of the ball rolling groove of the nut, that is, a surface existing outwardly in the diameter direction of the tube.